This casual bartering of "teaching" often triggers an annoyed reaction in me as well, and I'm not even a teacher here. I try to take it on as an opportunity to practice patience and even compassion. Often these messages are written in English by people with limited command of the language. Teach, exchange, converse, help, message, befriend, it's all in the same basket. 

And then there's the problem of attention. For many of us, minds flit as quickly as fingers across countless apps and exchanges in the course of a day. More, better, faster. Something's gotta give. There is a trade-off between speed and precision. Even for the people who are engaged in this discussion, people who are interested enough to consider the distinctions, there is time and mental energy involved and no promise of arriving at any definitive answer. Philosophy is out of fashion. Quick results are in demand. smb help!

It's true that in general passing on knowledge in one way or another is teaching. It's just difficult for me to differentiate what is meant by professional language teaching and just passing on knowledge. I don't think that users who say "I can teach you" are lying, not at all, but using the word "teach" just leads to a lot of misunderstandings. Whenever users write "I need somebody to teach me X language" teachers offers their services while the students were just looking for language exchange. And I think that I have just different expectations when somebody tells me that they can teach me a language. I then expect that they know the grammar and the subtleties of that said language. And have an idea where to start and how to help a non-native to progress. I think I have to overthink my own definition.

Thanks a lot for all those insightful comments! :)